The present invention relates to sheet feeding apparatus and in particular to a slide plate vacuum valve for use with a bottom vacuum corrugating feeding apparatus.
With the advent of high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, the need for a document handler to feed documents to the copy platen of the machines in a rapid, dependable manner was recognized to enable full utilization of the reproduction machines' potential copy output. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the originals and generate minimum machine shutdowns due to uncorrectable document misfeeds or document multifeeds.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of separators have been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member, if it acts upon the printed face, can cause smearing or partial erasure of the printed material on the document. With single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
In addition, a typical vacuum separation and feeding system is that described in U.S. Pat. No. 4,411,417 which is incorported herein by reference. Systems of that type use a solenoid operated butterfly-type vacuum valve to cycle vacuum flow. These systems are a marked improvement over prior feeders, however, the solenoid operated valve is a source of machine failure and the friction belts of the system in U.S. Pat. No. 4,411,417 that are arranged to run over a vacuum plenum placed at the bottom of a sheet tray are a source of concern for belt slip. Also, the belts allow air from an air knife that is used to separate sheet one from sheet two to pressurize under the lead edge of sheet one, causing flutter. The belt holes also allow vacuum leakage at the lead edge, increasing the probability of more than one sheet being fed at a time.